1. Field of the Invention
This invention relates to serial manufacturing operation, and more specifically relates to a method of balancing a serial assembly line.
2. Description of the Related Art
Certain types of manufacturing operations are normally organized into what are commonly referred to as assembly lines, manufacturing lines, or production lines. In this discussion, they will be referred to as assembly lines. Assembly lines operate as follows: Materials and parts that go into the final product are introduced to the line. Various manufacturing operations, inspections, tests, and/or handling operations are performed at workstations along the line in a serial manner. The result of these steps is the emergence of the final product at the end of the line.
A classic example of a progressive assembly line is that of an automobile assembly line. In this example the chassis, for instance, of the automobile to be assembled is conveyed along the line, passing the various operators at their workstations as it moves along. As the chassis passes a given workstation, the operator performs his given task, which may be, for example, to attach a fender to the chassis. This task is repeated for each chassis which is conveyed past the operator. Note that the operator performs only this task, and performs it on every chassis that goes past him. In modern assembly plants, the operator may be an automated machine or robot which performs the task on a repetitive basis. The line is designed such that the chassis is conveyed past the operator at such a speed that the operator has adequate time to perform his task properly. Equally important is the criterion that the chassis not be conveyed so slowly that the operator has excessive periods of idle time between operations.
Several terms must be defined in the context of this discussion. Although these terms have general meanings, their definition is more narrowly construed as presented herein. In this discussion the term task is defined as a minimum rational work element, i.e. an indivisible element of work, or a natural work element. A task may actually consist of several distinct actions, however further dividing the actions would cause unnecessary work in the form of extra handling. As an example, attaching a fender to an auto chassis could involve the actions of positioning the fender, fetching retaining bolts from a storage bin, placing the retaining bolts in the appropriate holes, fetching a tool to drive the bolts home, and finally driving the bolts home. Although this task as described consists of several distinct actions, to divide the task between operators would involve a great deal of unnecessary coordination between the two, and probable repetition of effort, hence the actions are grouped as an indivisible task. Task time is defined as the amount of time it takes to complete the task.
A workstation is a physical location at which a process, operation, test or inspection is performed or carried out on the in-process product. The workstation may be the location of a specific machine, a group of machines, or an operator and a machine. One task, or a plurality of tasks can be performed at a workstation. Generally each workstation requires one operator. However, in some cases, a single operator can operate several workstations, if time and physical constraints allow it. Conversely, in some cases (such as heavy machinery manufacture or aircraft assembly lines) several operators may be required to man a single workstation.
Another important term is the assembly line cycle time. The cycle time is a function of the line speed, and is the elapsed time between successive units moving along the assembly line. Another way of expressing this is that the cycle time is the amount of time the product is available at each workstation.
Delay time is a concept which is the result of uneven distribution of task times for the various workstations. To illustrate, consider a hypothetical assembly line consisting of four workstations, specified as W1, W2, W3, and W4, at each of which one task is performed, specified as T1, T2, T3, and T4 respectively. The cycle time for this line is ten seconds, meaning the product is available for processing at each station for ten seconds. Assume it takes ten seconds to perform T1, four seconds to perform T2, seven seconds to perform T3, and ten seconds to perform T4. The total time for a product to complete the assembly line is forty seconds (four stations times a ten second cycle time). The total of the actual task times, however, is thirty-one seconds. In this case, the delay time, is the difference between the total time for the product to complete the assembly line, and the total time required for the actual processing of the product while on the assembly line.
Delay time is often expressed as a percentage, and expressed as balance delay. To do so, the sum of the task times (thirty-one seconds) is subtracted from the total available line time (forty seconds); the result is divided by the total available line time, and multiplied by 100. In this case the balance delay time equals 22.5 percent. In other words, during 22.5 percent of the time that the product is moving along the assembly line, the operator(s) are idle and non-productive. If each task time was ten seconds, and the cycle time remained ten seconds, then the balance delay would be zero, meaning each operator is operating at full capacity, with no idle time. This is, of course, an ideal case, which is seldom achieved in actual practice.
If the above example were modified slightly by changing the task time of task 3 (T3) to six seconds, from seven seconds, an example of assembly line balancing can be demonstrated. In this new instance, the total task time is thirty seconds, the total line time is unchanged at forty seconds, and the balance delay is equal to twenty-five percent. However, note that T2 and T3 can be grouped together at one workstation, without violating the need to stay below the cycle time. Now, there are three workstations, specified as W1, W2, and W4. T1 is still assigned to W1 and the total task time for W1 is ten seconds (as before). T4 is still assigned to W4, with a total task time for W4 of ten seconds. W2 now contains tasks T2 and T3, with a total task time of ten seconds for the workstation. In this case the balance delay has been reduced to zero, from twenty-five percent, by combining two tasks to one workstation. Note that as originally expressed, this assembly line could not have been optimized in this way. In the first situation combining T2 and T3 would have resulted in a workstation with a cumulative task time of eleven seconds, which would have violated the cycle time available. In real terms, this means the operator of workstation W2 would have had ten seconds in which to accomplish eleven seconds worth of tasks, which is not possible. However, when T2's and T3's cumulative task time is below the cycle time, combining the tasks results in one less operator/workstation required, and in less idle time.
In a very simplified way, the above discussion provides the motivation and the methodology for assembly line balancing. As shown, the goal of assembly line balancing is to optimize the performance and production of an assembly line. This is done by `balancing` the various workstations such that: 1) each workstation has that number of tasks which leads to a minimum of idle time due to an imbalance of the total task times among the various workstations; and 2) the variation in cumulative task times due to the distribution of the tasks among the workstations is minimized. As shown, assembly line balancing can result in the need for fewer operators or workstations without a decrease in the production rate. Alternatively, assembly line balancing can be used to derive a shorter cycle time (and hence an increased production rate) without adding operators or workstations. Often a combination of shorter cycle time and reduced workstations is the desired goal.
Assembly line balancing is often explained in terms of a packing analogy, that is the balancing problem is analogous to that of packing a given number of equally sized boxes with blocks of varying sizes. The boxes represent the set of workstation cycle times, and the blocks of varying sizes represent the varying task times of the assembly line. The goal is to pack the boxes as fully as possible with the blocks so as to minimize the empty space left over (which corresponds to idle time).
Some blocks have restrictions on what box they can go into, and where in the box they must go (precedence constraints). However, most blocks can be placed wherever they will fit most efficiently. The are three factors which can impact the degree of difficulty experienced and the degree of optimization which can be achieved. These are: 1) the distribution of the sizes of the blocks; that is the ratio of large blocks to small blocks, and how large the large blocks are. Generally, the more large blocks there are, the more difficult the packing problem becomes. 2) The size of the blocks compared to the size of the boxes. Relatively small blocks would be easier to pack than blocks which are large, relative to the size of the box. There is a restriction, however, that no block can be larger than a box. 3) The number of blocks which have some restriction as to where they can be placed. Naturally, the more blocks which have such restrictions, the more difficult the packing problem becomes.
The best solution to such a problem is to start with the restricted blocks first by placing them where they must go. In this way, there is no worry about coming across them later when their required space may already be filled. Next, the larger blocks are placed in the boxes, before the available large spaces are inefficiently wasted by filling them with smaller boxes. Gradually, smaller and smaller blocks are dealt with as the larger blocks are placed, until finally the small gaps and voids left in the boxes can be filled in with the remaining small blocks. Visualizing the assembly line process in this physical manner aids in understanding the concepts and the rationale for the techniques employed.